ONE-ELECTRON AND 2-ELECTRON REDUCTION OF QUINONES BY RAT-LIVER SUBCELLULAR-FRACTIONS

NAD(P)H-quinone (menadione, Trolox C quinone, and cr-tocopherol quinone) reductase activity of rat liver subcellular fractions was observed optically at 340-400 nm, and oxygen radical generation was demonstrated using the ESR spin trap, 5,5'-dimethyl-1-pyrroline-1-oxide. NAD(P)H-menadione reductase activity of the fractions decreased in the order: cytosol > microsomes > plasma membranes. Although more than 65% of the activity of microsomes and plasma membranes was inhibited on the addition of dicoumarol, no change in the menadione-mediated formation of oxygen radicals by either fraction was observed. As judged from the intensity of ESR signals, the menadione-mediated oxygen radical formation by plasma membranes was only one-tenth as great as that by microsomes. No generation of oxygen radicals in the NAD(P)H-menadione reductase reaction by cytosol was found, and the activity was abolished in the presence of dicoumarol, an inhibitor of DT-diaphorase. It is concluded that plasma membranes reduce quinones by way of two-electron transfer and that the activity may prevent cellular quinone toxicity. NAD(P)H-alpha-tocopherol quinone reductase activity was confirmed in all cellular fractions [Hayashi et al. (1992) Biochem. Pharmacol. 44, 489-493] and this activity was also inhibited by dicoumarol, suggesting that it was due to DT-diaphorase.